What happened
On 12 January 2009, a Boeing 737-73V, registration G-EZJK, was conducting a customer demonstration flight west of Norwich. The flight was intended to verify the serviceability of the aircraft following a maintenance period before it was transferred to a new operator. The crew, consisting of two pilots, was performing a series of scheduled checks, including a flight control manual reversion test.
During this test, the crew isolated the individual hydraulic systems by switching the flight control switches to the OFF position. As the co-pilot deactivated the switches to remove hydraulic assistance from the primary controls, the aircraft suddenly entered a severe nose-down pitch. The aircraft reached a pitch attitude of -2.81° and began descending at rates of up to 3,100 fpm.
In an attempt to recover, the commander rolled the aircraft to the left. During the maneuver, the descent rate accelerated significantly, peaking at 21,000 fpm. The commander maintained heavy backpressure on the controls while attempting to stabilize the aircraft. After the aircraft leveled its wings, the descent rate was arrested, though the aircraft continued to pitch 30° nose down at high speeds. The crew eventually re-engaged the flight control switches, restoring normal hydraulic power and control. The flight was abandoned, and the aircraft returned to Southend without further incident.
The investigation
The AAIB investigation examined the aircraft's maintenance history and the procedures used during the flight test. It was noted that the aircraft had recently undergone maintenance involving an adjustment to the elevator balance tab settings. The investigation also reviewed the cockpit voice recordings and the procedures followed by the crew during the manual reversion check.
Findings
- The investigation established that the aircraft was significantly out of trim due to an adjustment made to the elevator balance tabs during recent maintenance.
- This trim imbalance was the primary factor that initiated the pitch-down incident when hydraulic assistance was removed.
- Confusion between the two pilots during the recovery phase led the commander to believe hydraulic power had been restored when the switches had actually remained in the OFF position.
- The maintenance arrangement involved a complex structure of sub-contracted management and oversight between multiple parties.
- The commander had identified the trim issue during a previous flight but had not formally recorded the concern in the technical log.